The field of the invention is synthetically produced higher fatty esters and the present invention is particularly concerned with the production of fatty acid esters by reacting olefins, alkanols and carbon monoxide in the presence of a cobalt catalyst and a promoter.
It is known that fatty acids can be prepared by reacting olefins with carbon monoxide and compounds containing a replaceable hydrogen atom such as alkanols in the presence of a catalyst containing a metal of Group VIII of the Periodic Table of Elements and possibly also containing a promoter (J. Falbe, "Synthesen mit Kohlenmonoxid," Springer publishers, Berlin, Heidelberg, New York, 1967).
In a preferred variation of this invention, which is called alkoxycarbonylation, the conversion takes place in the presence of catalysts containing cobalt. The additional use of pyridine or of a non-orthosubstituted alkylpyridine as the promoter is found to be an especially preferred embodiment.
The state of the art of alkoxycarbonylation may be ascertained by reference to U.S. Pat. Nos. 3,507,891; 3,856,832 and 4,041,057, the disclosures of which are incorporated herein.
A significant problem relating to the homogeneously catalyzed alkoxycarbonylation reaction is an industrially simple and loss-free process of recovery of the relatively expensive cobalt from the reaction mixture in a form which permits its reuse as a catalyst.
In the processes described in West German published application No. 19 63 804 and U.S. Pat. No. 3,507,891, this problem is solved by fractionated distillation of the reaction mixture obtained by alkoxycarbonylation, the cobalt used as the catalyst being obtained together with the distillation sump product.
With repeated or continued use, however, such a procedure results in an enrichment of high boiling point organic contaminations in the distillation sump product, as disclosed in West German Pat. No. 921,988 and European published application No. 00 08 024. Thereby the amount of residue continually grows and thus it must be fully reprocessed from time to time.
Reprocessing of the distillation sump product by boiling with a carboxylic acid or with water, as proposed in West German Pat. No. 921,988, entails difficulties, particularly in process engineering due to the tough and adhesive consistency of such residues. As the result of using carboxylic acids, the process suffers from corrosion problems and when boiling water is used, complete reprocessing of the water phase on ecological grounds is required.
The extraction of the distillation sump product using CO.sub.2, C.sub.2 -C.sub.4 -paraffin, C.sub.2 -C.sub.4 olefin or a halogen hydrocarbon above the critical temperature and the critical pressure of these compounds, as proposed in European published application No. 00 08 204, represents a problem without a simple solution in view of the required pressure exceeding 100 bars. Such a procedure also suffers from the dangers involved in using easily volatile and easily flammable compounds which include the lower olefins and paraffins. When the extraction is carried out with halogen hydrocarbons such as CClF.sub.3, there is furthermore the danger of catalyst poisoning by the entrainment of halogen compounds into the alkoxycarbonylation reaction.
The requirement for reprocessing the distillation sump product can be circumvented by applying the method disclosed in U.S. Pat. No. 3,856,832 for catalyst recovery. In this method, a hydroesterification reaction is carried out in the presence of an excess of methanol as the esterification component and hydrocarbons are added to the completely reacted reaction mixture. Two phases are formed as a consequence. The lower phase contains at least 95% by weight of the cobalt used as catalyst and this lower phase can be fed directly back into the hydroesterification reaction.
The drawbacks of the method of U.S. Pat. No. 3,856,832 include its restriction to the production of methylesters and on the other hand the loss of the cobalt remaining in the upper phase which has a concentration up to 5% by weight.
The method of U.S. Pat. No. 4,041,057 achieves complete cobalt recovery by recovering the cobalt which in the method of U.S. Pat. No. 3,856,832 remains in the upper phase as a distillation sump product. The tarry residue so obtained is burned at temperatures of 1000.degree. to 4000.degree. F.
The cobalt oxide so formed is filtered out of the combustion gases, and the cobalt oxide is converted into cobalt carbonyl by reaction with carbon monoxide and hydrogen. This cobalt carbonyl is then fed back into the hydroesterification reaction.
While a complete recovery is possible in the combination of the methods of U.S. Pat. Nos. 3,856,832 and 4,041,057, the efficiency and profitability of such an overall process, however, remains considerably reduced by the use of substantial amounts of hydrocarbons which substantially load the distillation reprocessing and by a costly, multistage recovery of the residual cobalt contained in the upper phase.